1. Field of the Invention
The present invention relates generally to prosthetic joints and, more particularly, to a modular prosthetic joint construction which enables the revision of components having a broad range of sizes for long bones with a minimum inventory of prosthetic components.
2. Description of the Prior Art
While the present invention is applicable for use with implants of various types and in numerous applications in human and animal joints, it will be described herein, for purposes of example only, as being specifically adapted for use in regard to a knee joint prosthesis.
The knee joint basically consists of the bone interface of the distal end of the femur and the proximal end of the tibia. Appearing to cover or at least partially protect this interface is the patella which is a sesamoid bone within the tendon of the long muscle (quadriceps) on the front of the thigh. This tendon inserts into the tibial tuberosity and the posterior surface of the patella is smooth and glides over the femur.
The femur is configured with two knob like processes (the medial condyle and the lateral condyle) which are substantially smooth and articulate with the medial condyle and the lateral condyle of the tibia, respectively. The condyles of the tibia are substantially smooth and slightly cupped thereby providing a slight receptacle for receipt of the femoral condyles.
When the knee joint is injured whether as a result of an accident or illness, cartilage covering the natural bones may become damaged to the extent that they are unable to function (articulate) properly. If the bones are affected beyond the level or degree where natural healing and new growth will remedy the damage, then a prosthetic replacement of the damaged portion is called for in order to relieve pain and to restore normal use to the joint. Typically the entire joint is replaced by means of a surgical procedure which involves removal of the ends of the corresponding damaged bones and replacement of these ends with prosthetic implants. A typical such implant would be for the hip joint wherein a metal implant could be anchored in the intramedullary canal of the femur and would provide a generally spherical protuberance extending outwardly therefrom. The mating prosthetic portion would be a polyethylene socket member suitably anchored into the acetabulum. While prosthetic devices of this type, normally including a physiologically inert metal member and an engaging high density polyethylene member, are well known in the art, these types of devices are of a fixed and unchanging nature once they are inserted (implanted) into the patient and anchored there, whether by pinning or by acrylic bone cement or both.
Due in part to the fact that the size, shape and anatomy of virtually every patient is different, great care must be taken by the orthopedic surgeon in order to select properly sized and shaped prosthetic members for implanting. In order to achieve a suitable fit and size compatibility, an extensive number of a variety of each type of prosthetic implant must be available to the orthopedic surgeon from which he may choose. As a result, the cost of inventory as well as the logistics of ordering and storing a wide variety of prosthetic implants is cumbersome. Therefore, it would be an improvement to this present situation if prosthetic devices could be structured with removable portions such that there would be a reduction in inventory without a corresponding compromise as to the number and variety of different prosthetic combinations which can be created. In order to provide such an improvement, it is necessary that the prosthetic portions which are to be mixed and matched into a variety of combinations be suitably secured together so that the prosthetic member, which they in combination create, is not weaker nor more likely to fail than would be a similar prosthetic member constructed as a single integral piece.
A further concern involves the procedure when a prosthetic device becomes worn or damaged and a replacement must be made. While this is possible, it oftentimes involves elaborate surgery depending upon the particular portion of anatomy involved and the extent or nature of the damage to the prosthetic device. Furthermore, certain portions of the anatomy such as knee joints may be more susceptible to wear due to the surface area of articulation, the nature of the anatomy and the typical loads and forces which are encountered by this particular joint. Repair and/or replacement may also be desired when interfacing portions of the anatomy change and the contacting portion of the prosthesis needs to be revised as to its shape or size.
By structuring a prosthetic implant such that the portion most likely to wear or desired to be changed is quickly and easily replaceable from the remaining portion of the prosthetic implant, significant amounts of surgical time can be saved and the prosthesis can be more closely tailored to the patient's needs. Equally important is the fact that the portion of the prosthesis which is anchored into the patient, such as a tapered shaft inserted into the intramedullary canal of the tibia or of the femur, does not have to be surgically removed in order to make a replacement of a worn or damaged tibia articulation portion. Such a replaceable concept, in order to be effective, must securely hold the anchored portion and the replaceable portion together so as to act as an integral member regardless of the nature or complexity of the forces and loads acting thereon. With a design which achieves the requisite strength and durability, it is then possible to mix and match the replaceable portion with various anchored portions and vice versa such that, for example, an inventory of five relatively large and expensive replaceable portions and five relatively small and inexpensive anchoring portions for a tibial or femoral implant would be able to provide 25 different combinations of each to the orthopedic surgeon rather than having to inventory and stock 25 separate complete relatively large and expensive portions.
Tibial and femoral primary or revision prostheses that are presently being marketed have a fixed central stem, located on the distal plane of the tibial tray, or component, and on the proximal portion of the femoral component's anterior/posterior box. This stem is used primarily for purposes of stabilization and strength, both during installation and after the joint has been rebuilt. It is difficult to determine prior to surgery, even by x-raying, the size of the stem to be used. As a result, it is often necessary to wait until the bone structure is personally viewed by the surgeon during the implant operation before selecting the proper size of the implant and, particularly, the stem.
In the past, it has been necessary for hospitals to maintain a large inventory of sizes of prosthetic joint components in order to accommodate different sizes of patients. However, the cost of maintaining such a large inventory is substantial. This expense sometimes becomes so great that a hospital will maintain no inventory, but will order a particular prosthesis only when needed. As a result, if elective surgery is to be performed, oftentimes components are ordered specially for the surgery to be performed which includes one size larger and one size smaller than is expected to be used. Revision surgery in cases of traumatic injury could not be performed since there would be no inventory on hand to provide prompt availability for all possible patients.
Typical of the prior art known to the inventor which is broadly of interest when considering the invention are the Noiles, U.S. Pat. No. 4,219,893 and Volz, U.S. Pat. No. 4,257,129. the Noiles patent discloses a prosthetic knee joint of the hinged type which permits rotations of the bones in two planes. In respect to the tibial component, a tibia stem whose upper part is rotatably engaged with the femoral component has a lower vertically depending rod which is rotatably received within a tibia sleeve which, in turn, is adapted to be implanted in the intramedullary canal of the tibia. The Volz patent is merely representative of a wide range of patents which disclose a tibial component having a downwardly extending stem for anchoring it to the tibia. A plastic bearing member is dove-tailed into receptive engagement on the upper surface of a support shelf and is held against further movement, once installed, by means of a vertical pin member which is received through the bearing and into a longitudinal bore formed in the stem and extending longitudinally in an axial direction. In neither patent is there a suggestion that the construction would enable the provision of various lengths and diameters of stems.
It is also known, in the instance of hip prostheses, for hospitals to inventory a number of femoral components for a hip prosthesis having a range of different sized heads for articulation in associated acetabular bearing components having a similar range of sizes. In some instances, stem extensions have been utilized in order to more readily accommodate a wide range of body sizes.
Although the foregoing devices have generally satisfied the goals for which they were intended, none offered the simplicity and economy which represent goals which have been achieved by the present invention.